There has been in recent years an increased demand for power factor correction and control in utility and industrial power systems due to the growing use of electrical machines, the major role of the electric arc furnace in steel production and the general acceptance of thyristor drives and power controllers in the industry. As a result, controlled generation of reactive power for improving the line power factor has become of major importance, which can be achieved most successfully through the use of static power switches, for instance thyristors. It has been established that static VAR generators in most applications provide superior performance at lower cost than conventional rotating synchronous condensers.
There are three basic modern methods of generating reactive power (VAR) all using static control of thyristors: (1) thyristor controlled shunt capacitors and inductors; (2) AC/DC converters and inverters; (3) AC/AC frequency changers.
The present invention relates to the third category of static VAR generators, namely to AC/AC frequency changers used for the generation of reactive power.
The AC/AC frequency changer is itself divided in several categories according to the mode of control, the range of control, and the inherent properties of the apparatus in operation. A first distinction is made between frequency changers in which the thyristors are force commutated and those in which the thyristors are naturally commutated by the voltages of the input source. Two inherent characteristics of a frequency changer using thyristors for conversion are: (1) the frequency relation between the input alternating current and the output alternating current (2) the phase relation between current and voltage at the input and at the output e.g. the displacement power factor. In converters of the naturally commutated type the displacement power factor of the current drawn at the input is lagging and is a function of the displacement power factor of the output current. It is possible by force commutation to control the input displacement power factor and in particular, to bring it automatically to unity. This is not possible with a naturally commutated frequency changer. Another particularity of frequency changers is that an alternating current sinusoidal wave of a desired frequency is generated by controlled conduction of the thyristors. The time and frequency of conduction of the thyristors is generally variable along the reference waveform used to build the output waveform. In one case, however, equal time intervals of conduction are caused by force commutation to occur in an array of thyristors and the derived output frequency is equal to the difference between the switching rate of the thyristors and the frequency of the input current. This particular type of frequency changer is called an Unrestricted Frequency Changer (UFC) because any output frequency can be generated for any input frequency merely by selecting the switching rate, whereas in the other types of frequency changers, the output frequency has to be lower than the input frequency, although with one exception, the naturally commutated cycloconverter in a particular instance when it is purely reactively loaded.
It has already been proposed to use a frequency changer in order to generate corrective reactive power to a power system. See for instance U.S. Pat. No. 3,858,105 entitled "Static Power Conversion Arrangement and Method" of L. Gyugyi.
It is desirable to be able to generate reactive power with apparatus of the appropriate rating as well as with due regard to technical requirement and cost objectives. In that respect, all reactive power generators in the prior art under any of the three basic methods have required the use of apparatus having a rating at least equal to the maximum VA power required for effective displacement power factor correction on the utilization line. In other words, the utilization of the reactive power sources of the prior art is one per unit (1 P.U.).
An object of the present invention is to provide a reactive power generator capable of compensating the power factor of a line with a VA rating which is less than the required reactive power.
Another object of the present invention is to provide an AC/AC frequency changer having an effective power rating which is larger than its inherent VA rating.
An object of the present invention is also to generate either inductive, or capacitive, reactive power from an AC/AC frequency changer as required to compensate for power factor displacement in a utilization line.
Static frequency changers offer a unique mode of generating reactive power. In that respect they provide an interesting alternative for AC/DC converters, and for inverters, which also are capable under proper operative conditions of generating reactive power. In all such instances, practically the reactive power generator must be operated in an essentially balanced multi-phase system. However, these types of generators not only perform generally as well as any rotating synchronous condensers under steady state conditions, but also have proved to be superior for transient response.